Boiling water nuclear reactors generally comprise steam-generating plants wherein reactor water coolant is circulated through a core of heat-producing fissionable nuclear fuel to transfer thermal energy from the fuel to the coolant, thereby generating a two-phase steam-water mixture emerging from the fuel core. Using steam-water separators and steam dryers positioned downstream from and above the core, the upward-flowing mixture from the heating core becomes partitioned into its respective phases, whereupon the steam is piped from the reactor vessel for use in steam-driven turbines or other equipment while the liquid water phase is recycled as coolant water.
In typical boiling water reactors used for power generation, reactor coolant water is circulated continuously around a flow path as follows: up through a heat-producing fuel core; then up through a core outlet plenum superimposed above the fuel core which serves to collect and channel all the coolant passing up through the fuel core; then on through an assembly of steam separators positioned above the core outlet plenum; and finally back downward outside of the core, along an annular region (termed the "downcomer") to recycle the liquid coolant and return it to the fuel core. When the reactor is designed as a natural circulation type boiling water reactor, this final flow path outside the core is direct and uninterrupted. A cylindrical member enshrouding the core and extending some distance both above and below the elevations containing the fuel core is positioned between upward-flowing water passing through the reactor core and downward-flowing water recycling back to the lower plenum beneath the core. If the reactor type is a forced circulation reactor, some form of pumping mechanism is positioned outboard of the core shroud member along this portion of the flow path to amplify the pressure head otherwise present in the reactor lower plenum region.
As the coolant passes through the fuel core, heat is transferred from the fuel assembles to the circulating water coolant. The water coolant emerges from the heat-producing fuel core as a two-phase mixture of steam and water, the proportions of which vary depending on such factors as the power output from the fuel, the amount of subcooling present in the feedwater, the total hydrodynamic flow resistance presented by the fuel core structure and its wetted surface, and the amount of orificing representing restrictions to flow immediately prior to the entrance of the coolant into the individual core fuel assemblies.
Conventional fuel assemblies of boiling water reactors are composed of a multiplicity of fuel units, such as rods grouped together in bundles, with each bundle surrounded by an open-ended channel through which water flows longitudinally. These channeled bundles of fuel units are spaced apart from each other to provide intermediate spaces for insertion of control blades. Thus, there are ample areas for coolant bypass flow beyond close proximity to the heat-producing fuel units within a bundle.
Bypass flow coolant water passes through the fuel core without closely encountering the high energy emanating from the fuel and enters the core outlet plenum consisting substantially of saturated liquid with perhaps a small amount of steam. This bypass effluent joins the two-phase steam-water mixtures exiting from individual fuel assemblies comprising the core. These two effluents, with differing ratios of steam contents and, in turn, buoyancy, rise up within the core outlet plenum from the core at different rates. Additionally the steam exit qualities tend to be higher from the central region of the fuel core than from the peripheral region of the fuel core.
Inconsistencies in the steam mass flow and, in turn, buoyancy and rising flow rates produce turbulences and eddies transversely across the moving stream of the mixture progressing upward within the core outlet plenum. This phenomenon of turbulence and eddies within a nuclear reactor is the subject of an article entitled "Peculiarities of the Distribution of Phases in the Updraft Section of a Housed Boiling Reactor" by V. N. Fedulin, G. G. Bartolomei, V. A. Solodkii and V. E. Shmelev, translated from Atomnaya Energiya, Vol. 57, No. 6, pp. 385-388, December, 1984, Plenum Publishing Corp. In addition to the turbulence and eddies of the uneven two-phase mixture of steam and water rising from the fuel core up through the core outlet plenum, the density of this mixture of gaseous steam and hot water within the core outlet plenum can be less than 25% that of room-temperature water.